U.S. patent application "Apparatus For Determining Thermal History of Equipment Using Solid State Track Recorders", Ser. No. 07/479,060, filed Feb. 27, 1991 now U.S. Pat. No. 5,064,605, that is a continuation of Ser. No. 07/479,060 filed Feb. 1, 1990, abandoned, and assigned to the assignee hereof. The application "Method for Determining An Equivalent Average Temperature Associated With A Thermal History of Equipment", Ser. No. 07/481,040 filed Feb. 16, 1990, is abandoned concurrently with the foregoing application and also assigned to the assignee thereof are concerned, as is this application, with techniques for detecting the thermal aging of equipment.
The properties of many materials change as a result of exposure to temperature. Rubber gaskets may take a permanent set, flexible members may become brittle, organic fluids may decompose and the electrical properties of insulating materials may vary. "Thermal aging" is the reason.
Safety equipment in nuclear power plants are required to be capable of performing properly under specified conditions, including theorized design events. Assurances are often required that the equipment will meet design performance characteristics throughout the installed life of the equipment. Those characteristics can be established through careful design qualification processes and surveillance testing of the equipment.
The qualification process involves establishing the qualified lifetime for the equipment. The qualified lifetime is the period of time for which the equipment has been demonstrated to meet the design requirements for specified operating conditions. The qualified lifetime of equipment may be extended if it can be shown that the service or environment conditions that were originally assumed were overly conservative. Substantiating the reason for an extension requires the application of specific documentation methods, as set out, for example, in section 6.9 of ANSI/IEE-323-984.
The qualification process often requires measuring the temperatures of highly localized equipment rather than the temperature of the environment. Some examples of this are a valve motor operator or limit switch that is heated by heat conduction up from the valve and valve yoke from a hot process fluid (e.g. a steamline) controlled by the valve. Another example is a solenoid valve that is heated by resistance heating when the coil is energized, in addition to the heat produced from a hot process fluid line. The equipment itself is significantly hotter than the area ambient temperature in these examples. In yet another application for these devices, electrical cable is placed in a cable tray and the temperature of the cables is monitored. The cables may sustain substantial temperature elevations due to IR.sup.2 losses, depending on the thermal characteristics of the cables and the tray. Fireproofing materials that coat the tray can increase these temperatures.
Methods of taking passive radiation and temperature measurements involve the use of large holders for the temperature monitor. These are not easily mounted directly on the equipment under test. Furthermore, the mass of the monitor, as well as its geometry make contact measurements rather difficult and complex. U.S. Pat. Nos. 4,879,058, 4,490,118 and 5,064,605 discuss this.
One method for passive monitoring of a temperature environment, described in U.S. Pat. No. 4,167,109, involves the use of a solid state track recorder (SSTR). There it is explained that by determining the extent of annealing of the radiation "tracks" in the SSTR, the temperature to which the SSTR has been exposed can be inferred. In general, a passive temperature measuring device may be created by exposing a member, formed from an appropriate material, to an altering agent. Such exposure produces an alteration in the member which undergoes a quantifiable change as a result of exposure to temperature (i.e., the member thus altered is subject to thermal aging). An SSTR is such a passive temperature measuring device. Basically, an SSTR is a member formed from a material, generally a dielectric material, in which, exposure to energetic charged particles results in the formation of observable "tracks" which anneal as a result of exposure to temperature. Basically then, an SSTR is utilized by exposing it to energetic charged particles, such as fission fragments or alpha particles, from a radiation source. The passage of these particles through the SSTR produces a permanent trail or radiation damage along the trajectory of each particle, termed a "track. " As a result of this radiation damage, the track is subject to preferential attack to an etchant, thus rendering the track visible upon magnification. If the trajectory of the track were normally incident to the surface of the SSTR, the track would appear as a small round pit in the surface. A type of monitor can consist of different SSTR elements, an assembly often called an a integrating thermal monitor or "ITM".
In an SSTR based monitor, the radiation damage associated with the track reverses itself with exposure to temperature, a thermal aging process referred to as annealing. As a result of this phenomenon, annealed tracks, observed after etching, are smaller. Hence, this process is sometimes referred to as "track fading." Moreover, since the reduction in track size renders some tracks invisible, the density of the tracks is also reduced. Hence, the extent of track annealing (thermal aging) may be quantified by determining the reduction in the average track diameter or the reduction in the track density. Calibration standards are created for each SSTR material by forming a number of test members from the material and exposing these test members to energetic charged particles. The tracks thus formed in the test members are then subjected to annealing at a variety of temperatures for a variety of times.
The Arrhenius function has been used in the past to analyze the results of accelerated thermal aging tests, for example as disclosed in S. Carfagno and R. Gibson, A Review of Equipment Aging Theory and Technology, EPRI Report, NP-1558, .sctn.8.3 (1980), and to analyze data from naturally occurring particle tracks in naturally occurring glasses (e.g. used in SSTRs) for geological dating purposes, for example as disclosed in D. Storzer, Fission Track Dating of Volcanic Glasses and the Thermal History of Rocks, in Earth and Planetary Science Letters, 8, pp 55-60 (1970).
In the Arrhenius function, the value of the activation energy E depends on the particular process. Thus, there are potentially an infinite number of Arrhenius functions, each characterized by a different value of the activation energy term. By varying the value of the activation energy term, an individualized Arrhenius function can be developed to characterize the temperature dependence of the reaction rate for any given process.
The processes associated with thermal aging of equipment can be characterized by Arrhenius functions, as evidenced by the fact that the Arrhenius model has often been used in accelerating aging tests (to extrapolate the results of short time exposure at high temperatures to obtain expected aging effects due to long time exposure at lower temperatures). Thus, the thermal aging process in the equipment to be monitored, for example, a piece of qualified equipment in a nuclear power plant, can be described in an Arrhenius function, characterized by an activation energy which represents the effect of temperature on the rate at which the particular aging process occurs. For example, 50% compression set in a gasket material might have an activation energy of 1.2 eV, whereas a 25% loss in lubricity of lubricant might have an activation energy of 0.8 eV. The activation energy values in this example, which have been arbitrarily assigned for illustrative purposes only, would indicate that the rate of lubricity loss increases more rapidly with increasing temperature than does the rate of gasket compression set.